- consistent treatment of errors in command line arg parsing (exit only when actually processing a command line, not in inetractive mode or when initializing the api)
- automatic listing of available mesh formats

- removed every "throw;", i.e., every throw that did not throw an actual exception.
throw wihout args should ONLY be used in a catch{} to re-throw the same exception
(for it to be handled in another catch clause)
- introduced GmshMessage callback class so that clients can easily implement
their own message handling (see example in driverOCC.cpp)

did in high
* added mesh generation error log summary
* removed Parallel subdir (this is now handled by the message class)
* removed Box subdir and consolidated batch processing in a single
routine
* made List_T into a class
* reduced dependencies in Pview*.h so we can eventually add the post-pro
interface to the public API